If you sign up to Audible on their one month free trial, you can download the audiobook version for nothing. It's 32 hours in length! If you don't, you can still buy the audiobook via Amazon for £3.35! The Kindle edition is just £3.79.
But the physical book can be picked up pretty cheaply most places (including Amazon). The current edition that's just been released by Bloomsbury has an updated forward by Susanna that ties in with the BBC adaptation.
(I own so many copies of this book - I have it on Kindle, iBooks, plus the three editions of the printed book, and a very limited copy of one of the volumes that's still bound and tied with string and sealed with the Raven King's wax seal - sent to me by Susanna and Colin).